The disclosure of Japanese Patent Application Laid-open No. HEI 10-163764 filed on Jun. 11, 1998 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to an apparatus for detecting the resistance of an air-fuel ratio sensor and, more particularly, to an air-fuel ratio sensor resistance detecting apparatus for detecting the impedance of an air-fuel ratio sensor used to detect an exhaust gas air-fuel ratio, for example, an oxygen concentration detector element.
2. Description of the Related Art
In order to allow a catalyst disposed in the exhaust system of an engine to remove a maximum amount of harmful components (for example, hydrocarbon (HC), carbon monoxide (CO), oxides of nitrogen (NOx) and the like)as from exhaust emissions, recent air-fuel ratio control technologies employ an air-fuel ratio sensor disposed in the exhaust system, and perform feedback control such that the exhaust air-fuel ratio detected by the air-fuel ratio sensor becomes a target air-fuel ratio, for example, the theoretical air-fuel ratio. An air-fuel ratio sensor normally employed by these technologies is a limiting current-type oxygen concentration detector element that outputs a limiting current proportional to a concentration of oxygen in exhaust gas from the engine. The limiting current-type oxygen concentration detector element makes it possible to linearly detect the exhaust air-fuel ratio based on the detected oxygen concentration over a wide range. Thus, the limiting current-type oxygen concentration detector element is useful for improving the air-fuel ratio control precision, or for performing lean-burn control.
The oxygen concentration detector element requires that the element be kept in an active state, in order to maintain high precision air-fuel ratio detection. Typically, when the engine is started, the element is immediately heated by electrifying a heater attached to the element, in order to quickly activate the element. The electrification of the heater is then controlled so as to maintain the active state of the element.
FIG. 27 is a graph indicating the correlation between the temperature and the impedance of oxygen concentration detector elements. The temperature and the impedance of an oxygen concentration detector element (hereinafter, simply referred to as an xe2x80x9celementxe2x80x9d) normally have a correlation indicated by line A in FIG. 27, that is, a relationship in which the element impedance diminishes with an increase in the element temperature. Based on this relationship, the aforementioned heater electrification control detects the element impedance, and derives therefrom an element temperature, and performs a feedback control such that the derived element temperature becomes a desired activation temperature, for example, 700xc2x0 C. For example, if the element impedance Zac is equal to or greater than 30xcexa9 (Zacxe2x89xa730), i.e., the element impedance corresponding to the initial control element temperature 700xc2x0 C., that is, if the element temperature is equal to or lower than 700xc2x0 C., as indicated by line A in FIG. 27, the heater is electrified. If Zac is less than 30xcexa9 (Zac less than 30), that is, if the element temperature is higher than 700xc2x0 C., the electrification of the heater is discontinued. Through this control, the element temperature is kept equal to or higher than 700xc2x0 C., i.e., the activation temperature of the element, so that the active state of the element is maintained. During electrification of the heater, an amount of power supply needed to eliminate the deviation of the element impedance from the target value (i.e., Zacxe2x88x9230) is determined, and duty control is performed so as to supply that amount of power to the heater.
A method for detecting the temperature of an oxygen concentration sensor, disclosed in, for example, Japanese Patent Application Laid-open No. HEI 9292364, detects the impedance of the oxygen concentration detector element by applying a DC voltage for air-fuel ratio detection together with a superimposed AC voltage having a frequency suitable for detecting the element temperature, for example, 5 kHz, to the element, and measuring the current through the element after the AC voltage superimposition. Based on the superimposed applied voltage and the measured current, an element impedance is detected.
However, the element impedance detected by the aforementioned method for detecting the resistance of an oxygen concentration sensor element has the following problems. An oxygen concentration sensor disposed in an exhaust passage of an internal combustion engine undergoes aging deterioration of electrode portions of the element due to exhaust gas heat or deposit on interiors or surfaces of the electrodes of the element, so that the correlation between the element impedance and the element temperature changes as indicated by line B in FIG. 27. That is, as the sensor element deteriorates, the detected element impedance values deviate. Furthermore, oxygen concentration sensors disposed in exhaust passages also experience deviations in detected impedance values due to the changing exhaust gas conditions of depending on the intake air flow, the load condition of the engine, the exhaust air-fuel ratio, and the like.
Deviations of the detected impedance values as described above naturally cause undesired events. For example, even if the target impedance is 30xcexa9 and the present true element impedance is 30xcexa9, the element impedance may be falsely detected as 20xcexa9 due to a deviation as mentioned above, so that the element temperature is determined to be 800xc2x0 C. In that case, the heater is controlled so as to reduce the element temperature. If such control is continued, the sensor element is cooled below the activation temperature of 700xc2x0 C., thus failing to maintain the active state. As a result, the air-fuel ratio control precision deteriorates, and exhaust emission becomes degraded.
Furthermore, if the target impedance is 30xcexa9 and the present true element impedance is 30xcexa9, the element impedance may be falsely detected as 90xcexa9 due to a deviation as mentioned above, so that the element temperature is determined to be 600xc2x0 C. In that case, the heater is controlled to increase the element temperature. If such control is continued, the sensor element temperature exceeds the activation temperature of 700xc2x0 C., that is, the sensor element is overheated. As a result, deterioration of the sensor element is accelerated, and the service life thereof is shortened.
Accordingly, it is an object of the present invention to provide an air-fuel ratio sensor resistance detecting apparatus that prevents deterioration of the sensor element due to overheating thereof and prevents deterioration of a heater resistor due to excessive power supply thereto even when the sensor element deteriorates over time or when the condition of a detection object gas changes.
It is another object of the invention to provide an air-fuel ratio sensor resistance detecting apparatus that determines whether the air-fuel ratio sensor element has failed.
To achieve the aforementioned and other objects, one aspect of the invention provides an air-fuel ratio sensor resistance detecting apparatus including an oxygen concentration detecting element that detects an oxygen concentration in a detection object gas (e.g., exhaust gas from an engine), a heater for activating the oxygen concentration detecting element, an air-fuel ratio detection device for detecting an electric current through the oxygen concentration detecting element proportional to the oxygen concentration in the detection object gas by applying a voltage to the oxygen concentration detecting element, the air-fuel ratio detection device detecting an air-fuel ratio of the detection object gas based on the electric current. The air-fuel ratio sensor resistance detecting apparatus further includes a gas condition detection device for detecting a condition of the detection object gas, an impedance detection device for detecting an impedance of the oxygen concentration detecting element by applying a voltage to the oxygen concentration detecting element, and a correction device for correcting the impedance detected by the impedance detection device, in accordance with the gas condition.
This apparatus corrects the impedance of the oxygen concentration detecting element in accordance with the gas condition of the detection object gas detected by the oxygen concentration detecting element, and corrects the target impedance for the oxygen concentration detecting element. Therefore, the apparatus is able to properly control the target impedance in accordance with the gas condition and prevent overheating of the oxygen concentration detecting element and the heater.
In accordance with another aspect of the invention, an air-fuel ratio sensor resistance detecting apparatus includes an oxygen concentration detecting element that detects an oxygen concentration in a detection object gas, a heater for activating the oxygen concentration detecting element, an air-fuel ratio detection device for detecting an electric current flowing through the oxygen concentration detecting element, the current being proportional to an oxygen concentration in the detection object gas, upon application of a voltage to the oxygen concentration detecting element, an electric power calculation device for calculating an amount of electric power supplied to the heater, an impedance detection device for detecting an impedance of the oxygen concentration detecting element by applying a voltage to the oxygen concentration detecting element, and a correction device for correcting the impedance detected by the impedance detection device, in accordance with the amount of electric power supplied to the heater.
This apparatus uses the amount of electric power calculated by the electric power calculation device as a parameter corresponding to the deterioration of the sensor element (e.g., resulting from aging changes) to correct the impedance of the oxygen concentration detecting element and therefore correct a target impedance set for the oxygen concentration detecting element, in accordance with the amount of electric power. Therefore, the apparatus is able to properly control the target impedance in accordance with the deterioration of the sensor element despite aging changes and prevent overheating of the oxygen concentration detecting element and the heater.
The impedance detection device may detect the impedance of the oxygen concentration detecting element by applying a voltage obtained by superimposing an AC voltage on a DC voltage to the oxygen concentration detecting element. In that case, the impedance of the oxygen concentration detecting element can be detected within a short time.